1. Field of the Invention
The present invention is related to a gas constituent electrochemical sensor for sensing the concentration of oxygen in an unknown gas relative to a reference gas. This sensor has a tubular electrolyte thimble with an internal and external conductive coating connected to a control member. Changes in oxygen concentration in the gas as compared to the reference concentration creates an operational signal indicative of air/fuel ratio supplied to an engine. A heater member receives electrical current from the control member to maintain the temperature of the thimble above a minimum operational temperature. As a result, the operational signal is only dependent on the difference in the oxygen concentration between the unknown and reference gases.
2. Description of the Prior Art
It is known that a body of solid electrolyte, for example zirconium dioxide, which has one surface exposed to a reference oxygen concentration and a second surface exposed to an unknown oxygen concentration may generate an electrical potential between such surfaces. Examples of sensors which use such an electrolyte member may be found in U.S. Pat. Nos. 3,960,692 and 3,960,693, which issued June 1, 1976, U.S. Pat. No. 4,019,974 which issued Apr. 26, 1977 and U.S. Pat. No. Re. 28,792, which reissued Apr. 27, 1976, the disclosures of which are incorporated herein by reference.
By coating the surface of the zirconium dioxide, or other solid electrolyte body with a catalyst material, such as platinum, a relatively high output signal can be generated whenever the combustion mixture is at an air/fuel ratio less than the stoichiometric mixture ratio for that fuel and a relatively low signal whenever the mixture is at an air/fuel ratio greater than the stoichiometric mixture ratio for that fuel. Thus, a generally step function will be generated by the sensor as the air/fuel mixture ratio passes through stoichiometry from a relative low to high value.
Typically, the solid electrolyte, as illustrated in the above referenced patents, is formed in the shape of a closed ended tube or a thimble. The thimble is coated on the inside and outside with a porous metallic electrode material, for example, platinum or palladium. The exterior of the closed end tube or thimble is inserted into the exhaust system and exposed to the heated exhaust gas created by the combustion of gases within an internal combustion engine, or is exposed to the incoming air/fuel mixture, while the interior of the close ended tube or thimble is exposed to atmospheric conditions. Thus, the sensor generates a voltage that is proportional to the difference between the partial pressures of oxygen between the interior and exterior of the electrolyte thimble.
The outside surface of the thimble is usually connected to an electrical ground through the attachment of the housing of the sensor to the exhaust system. The exhaust system in turn is connected to the chassis of the vehicle to form one of the conductors for a sensor. This type electrical ground connection is illustrated by a clip disclosed in U.S. Pat. No. 4,111,778.
Most vehicles use a multiplicity of electrical loads and as a result the electrical ground conductor for an oxygen sensor may have various electrical potential applied thereto and is practically never completely free of voltage variations. Unfortunately, the output signal as measured across the inner surface terminal of the sensor and outer surface terminal grounded to the vehicle may not be the true output from the sensor itself. The electrical loading characteristics and operational potential generated by the other electrical components of the vehicle may therefore change the actual voltage signal as measured in the sensor and could interfere with proper operation of the control system setting of the air/fuel mixture to be supplied to the combustion chamber. Thus, the composition of the exhaust gases being emitted from the engine could be adversely affected by this type of sensor construction.
U.S. Pat. No. 4,019,974 attempted to overcome this problem with a positive ground crimped to a terminal connected to the outside surface conductor. In this sensor, a resilient conductive mass of graphite is positioned around the thimble by a mass of insulating powder to provide an electrical path between the outside conductive surface and terminal connector. Unfortunately this structure requires some complicated and expensive manufacturing in order to maintain the components in their spacial relationship to assure that an adequate and uninterrupted electrical flow path is produced between the outside conductive surface and the electrical terminal.